This invention relates to an article made of a nickel-base superalloy, and, more particularly, to the protection of the surface of such an article.
In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against an airfoil section of the turbine blades and vanes, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the gas turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are made of nickel-based superalloys, and can operate at temperatures of up to about 1800-2100xc2x0 F.
Many approaches have been used to increase the operating temperature limit and service lives of the turbine blades and vanes to their current levels. For example, the composition and processing of the base materials themselves have been improved. Cooling techniques are used, as for example providing the component with internal cooling passages through which cooling air is flowed.
In another approach, the surfaces of the turbine blades and vanes are coated with aluminum-containing protective coatings that protect the articles against the combustion gas, and in some cases insulate the articles from the temperature of the combustion gas. An example is overlay coatings applied to some or all of the surfaces of the article to be protected. A ceramic layer may overlie the protective layer. The articles are thereby able to run cooler and are more resistant to environmental attack.
Although all of these approaches are effective in improving the performance of gas turbines, there is an opportunity for additional improvements to extend the operating temperatures and service lives of the gas turbine components. There is a need for improved protective coating systems that extend the capabilities of the turbine components even further. The present invention fulfills this need, and further provides related advantages.
The present invention provides a nickel-base superalloy article protected by a protective layer, and a method for its preparation. The article is particularly useful as a gas turbine blade or gas turbine vane. The article has a prolonged life in the thermal cycling conditions found in aircraft engine operation, as compared with conventional articles.
A nickel-base superalloy article protected by a protective layer comprises an article substrate having a surface and comprising a nickel-base superalloy, and a protective layer on the surface of the article substrate. The protective layer comprises nickel, from about 20 to about 35 weight percent aluminum, and from about 0.5 to about 10 weight percent rhenium. The protective layer is preferably of a predominantly beta (xcex2) phase NiAl type, and is preferably from about 0.0005 inch to about 0.004 inch thick. The protective layer more preferably comprises from about 0.5 to about 7 weight percent rhenium, and most preferably comprises from about 0.5 to about 3 weight percent rhenium. Elements initially present in the substrate may and usually do interdiffuse into the protective layer. The protective layer may also include modifying elements such as zirconium, hafnium, chromium, yttrium, and/or silicon to further improve its properties. A ceramic layer may overlie the protective layer.
The addition of rhenium to the protective layer improves the stability of the protective layer because it reduces the diffusional rates in the nickel-base superalloy substrate and in the protective layer. The stability is also increased because rhenium has limited solubility in the gamma prime phase of the substrate and therefore the beta phase NiAl is retained for a longer period of time. The service life of the protective layer is thereby increased. The rhenium also strengthens the protective layer, improving its resistance to creep and other failure mechanisms.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.